Retreat Operations
Retreat operations can increase the size of the used data space in a NET_BUFFER structure or in all of the NET_BUFFER structures in a NET_BUFFER_LIST structure.
NDIS provides the following retreat functions:
NdisRetreatNetBufferListDataStart
Retreat operations can sometimes allocate MDLs that are associated with a NET_BUFFER structure. To provide the mechanism for allocating MDLs, a driver can provide an optional entry point for a NetAllocateMdl function. If the entry point is NULL, NDIS uses a default method to allocate MDLs. MDLs must be freed within a NetFreeMdl function that provides the reciprocal of the mechanism that was used to allocate the MDL.
To obtain the new DataLength, NDIS adds the driver-specified DataOffsetDelta to the current DataLength . If the size of the unused data space is greater than the DataOffsetDelta, a retreat operation reduces the DataOffset . In this case, the new DataOffset is the current DataOffset minus the DataOffsetDelta .
If the DataOffsetDelta is greater than DataOffset, a retreat operation allocates new data space. In this case, NDIS adjusts the DataOffset accordingly.
For send operations, NDIS allocates memory if there isn't enough unused data space to satisfy a retreat request. If no memory allocation is required, NDIS simply adjusts the DataOffset and DataLength . For better performance, drivers should allocate enough total data size before sending to accommodate the retreat operations of all the underlying drivers.
For the receive return case, NDIS simply adjusts the DataOffset and DataLength accordingly. The retreat operation reverses the advance operation that took place during receive processing. After the retreat operation, the used data space contains the header data that underlying drivers used during receive processing.